1. Field of the Invention
This invention relates to an obstacle sensing apparatus for a self-propelled cleaning robot, and more particularly to an improved arrangement of ultrasonic elements, which minimize the influence of ultrasonic directivity during cleaning operations of the self-propelled cleaning robot, thereby precisely sensing a position of an obstacle.
2. Description of the Prior Art
Generally, a self-propelled cleaning robot is of the type as shown in FIGS. 1 to 3 of the accompanying drawings, which comprises a body 1, driving and steering means 2 for moving the body, combined driving and steering wheels 3, auxiliary wheels 4, a power source, i.e., a battery 5, travel direction determining means 6, mounting members 9 disposed at both sides of the front portion of the body 1 and each made of a three-sided plate having a forwardly facing central portion and right and left side portions inclined inwardly at an angle of 45.degree. relative to the central portion, ultrasonic distance-measuring means including three ultrasonic elements 7a, 7b, 7c disposed at each mounting member 9 and ultrasonic distance-measuring circuits 8, 8, 8 connected one to each of the ultrasonic elements, and obstacle discriminating means for judging presence and absence of an obstacle on the basis of the output of the ultrasonic distance-measuring means.
Operation of the self-propelled cleaning robot thus constructed will now be explained with reference to FIGS. 4 to 7.
First, when the gleaning robot travels in a juxtaposed manner along a left side wall 100 from position P.sub.1 to position P.sub.s as shown in FIG. 4, the driving and steering means 2 is operated by the travel direction determining means 6 to control the posture of the body 1 in such a manner that if the ultrasonic element 7a of the ultrasonic distance-measuring means facing the side wall 100 senses the side wall which gives obstacle information as shown in FIG. 5, the body is turned to the right, otherwise the body is turned to the left. In this manner, the robot travels along the side wall 100 in the direction of travel of P.sub.1 to P.sub.s, while maintaining a parallel relationship to the side wall in response to sensing of the side wall (i.e., the obstacle) by the ultrasonic element 7a.
Then, when the robot has reached the position P.sub.s, as shown in FIG. 6, and the ultrasonic element 7c facing forwardly detects a new side wall 101 perpendicular to the left side wall 100, the robot first stops traveling, and the driving and steering wheels 3 are turned to the right by an angle of 90.degree. by the travel direction determining means 6 to turn the body to a position in which the left side ultrasonic element 7c cannot detect any side walls. Thus, the body 1 is positioned parallel to the new side wall 101, as shown in FIG. 6, and thus can again begin to travel along the new side wall. From this position, as shown in FIG. 4, the robot travels toward position P.sub.4 while keeping a parallel relationship to the new side wall in the same manner as the travel from P.sub.1 to P.sub.s as described above. Here, similar parts are denoted by similar numerals and actions of the respective constituent elements are not described further because the actions are the same as those in the travel from P.sub.1 to P.sub.s.
FIG. 7 is an explanatory view shoving sensing areas of the ultrasonic elements in operation of the ultrasonic distance-measuring means of the self-propelled cleaning robot according to the prior art. During traveling of the robot, when the ultrasonic distance-measuring means operates, the central ultrasonic element 7b can detect an obstacle existing within the range of about.+-.L.sub.l (about.+-.15 cm) from the central axis of the element. Further, each of the ultrasonic elements 7a, 7c disposed at the inclined side portions of each mounting member 9 can detect an obstacle existing within the range of the interior distance of L.sub.l from the central axis of the element of the mounting member 9 and a perpendicular line W.sub.l or W.sub.s passing through the center of the right or left ultrasonic element 7a or 7c) from the central axis. Therefore, the ultrasonic distance-measuring means can detect the obstacle within the range of 90.degree. which is the angle that the right side inclined portion of the mounting member 9 makes with the left side inclined portion of the member. Each of the ultrasonic elements 7a, 7b, 7c used for the detection of the obstacle is of the horn type.
The prior art cleaning robot as described above is disclosed in Japanese Laid-Open Patent Publication HEI 2-24142 (the applicant: Matsushita Electric Company), the contents of which are incorporated herein by reference.
In the ultrasonic elements applied in the prior art cleaning robot as discussed above, the intensity of a sonic wave is highest in the forward direction, but becomes weaker in both lateral directions because of ultrasonic directivity. As used herein, the term "ultrasonic directivity" means that since an ultrasonic wave does not have a straight traveling property, its sensitivity varies depending upon direction, so that precise distance determination may not be accomplished.
More specifically, since an ultrasonic signal has greater amplitude at a short distance and less amplitude at a long distance, as shown in FIG. 8, when the ultrasonic wave is emitted, without being directed to an obstacle as indicated by arrow A (assuming that the obstacle is located in the distance beyond the sensing area of the ultrasonic element), the ultrasonic element 7c is affected by the ultrasonic wave of greater amplitude from the ultrasonic element 7a that is, from the ultrasonic directivity as indicated by arrow B. Accordingly, the cleaning robot may mistakenly determine the distance of the obstacle as being closer than the actual distance. As a result, during traveling, a malfunction of the robot or a breakdown of the robot due to a collision with the wall may take place.